In this context, it has proved to be advantageous to use static frequency converters instead of rotary converters because static frequency converters have clear advantages over rotary converters with respect to dynamic, wear, weight and availability. Frequency converters have to generate a sinusoidal symmetrical alternating current voltage that, in addition, has a distortion factor of less than 5%. For testing the insulation systems of power transformers, induced voltage testing is used in which partial discharge measurements are carried out. If highly inhomogeneous field distributions occur in high-voltage insulations or along air gaps, this can result in local exceeding the material-dependent breakdown field strength. In this state of an incomplete electrical breakdown, the insulation between the electrodes is only partially bridged by discharges. Such partial discharges occur primarily when the insulation is subjected to alternating current voltage. However, for a successful partial discharge measurement, the external acting interference must not exceed a partial discharge interference level of 100 μC. In order to ensure this, the respective outputs of the frequency converter are fed to a sine-wave filter that acts as low-pass filter.
However, a static frequency converter inherently generates a pulse-width-modulated rectangular output voltage with a high distortion factor that in this form is not suitable for testing power transformers. A sine-wave filter arranged at the output of the static frequency converter filters the fundamental wave between the outer conductors of the converter output and thus suppresses the symmetrical interference generated by the converter. Despite the additionally provided sine-wave filters, frequency-pulsed jumps of the conductor-ground voltage at the is converter output cannot be suppressed. These undesirable asymmetrical interference are conducted via different electrical coupling mechanisms far into the transformer being tested and cause considerable interference therein for the intended partial discharge measurement.
From the post-published application DE 10 2007 059 289 [US 2010/0259280] of the applicant, a generic test apparatus became known that suppresses the problem of asymmetrical interference by providing a static frequency converter that has a plurality of outputs connected to a filter whose outputs in turn are connected to a matching transformer that itself is connected to the test object, in particular a transformer, provided for the actual test. In this application it is proposed that the filter be a transformer that has an electrostatic shield between its primary and secondary sides and whose secondary-side outputs are connected to filter capacitors are connected together in a star connection with a grounded center. This arrangement for partial discharge measurement is possible in DE 10 2007 059 289 because the provided filter capacitor effects a potential separation on the load side between the frequency converter and the installed filter capacitors. In case of conventional sine-wave filters only composed of inductors connected in series and capacitors, the filter capacitors cannot be arranged in this manner because otherwise a capacitive converter phase-to-ground fault would occur.
However, as apparent from company publication 8.71/3, efforts have been made for some time to offer the customers mobile transformer testing systems so as to allow “on-site testing” of is high-power transformers with the systems. These mobile testing systems are installed in a container as prefabricated units and are driven with a transport carriage to the testing site or the location of the test object. On site, the completion of the finally required test circuit is possible in only a few steps. The benefit for the customer is the mobile availability of a single testing system for testing a plurality of permanently installed high-power transformers in different locations. The savings with respect to installation time and operating costs of such a mobile testing system are high compared to conventional stationary testing units. Thus, there is a correspondingly high demand for mobile testing systems that can be dimensioned in a more variable and stable manner with respect to the range of their testing capacities. However, the dimensioning with respect to the testing capacity is directly related to the total weight of the testing system. Since the latter, as already mentioned, is installed in a container, the total weight is governed by the road traffic regulations applicable in Germany and thus by legal restrictions with respect to the permissible total weight as well as the axle load distribution. In the international environment, there are similar restrictions with almost identical limit values. The mobile testing system known from company publication 8.71/3 exhausts the limits of the legally permissible total weights and axle load distributions to the greatest possible extent. The filter transformer installed in this model for suppressing asymmetrical interference has a capacity of 500 kVA. A significant increase of the testing capacity cannot be achieved with such a testing system.